The MSRV is now 1.54.0, because reddsa 0.2.0 included a fix to its
nightly CI that inadvertently bumped its MSRV.
The `halo2` crate is now the `halo2_proofs` crate, but we're avoiding
the cross-repo crate rename until after `halo2_gadgets` is extracted.
This also brings in the 20% prover performance improvement from
zcash/halo2#447.